B. G. Pokrovsky

Constraints on 87Sr/86Sr of Late Ediacaran seawater: insight from Siberian high-Sr limestones

Abstract: In SE Siberia, carbonate formations with δ13Ccarb values ranging between −12‰ and −7‰ (V-PDB) and Sr concentrations of up to 2.5% occupy an area of 40 000 km2. Several successions exceed 1000 m in thickness and represent the worlds largest known exposures of sedimentary carbonates exhibiting extreme depletion in 13C. The carbonates were deposited on a carbonate platform evolving from a mixed carbonate–siliciclastic ramp to a carbonate ramp, and then from a peritidal rimmed shelf to a deep-water open shelf. All sequences reveal a facies-independent, upward rise in marine δ13Ccarb from −12‰ to −7‰. The trend and magnitude of the values mimic those that are characteristic of the 600–550 Ma Shuram–Wonoka isotope event. A coincident stratigraphic rise in 87Sr/86Sr from 0.70802 to 0.70862 in several sections of limestones, containing 4400 μg g−1 Sr on average, is considered to be by far the best available constraint on a temporal variation of seawater isotopic composition through the Late Ediacaran. If the greatest temporal rate of change in seawater 87Sr/86Sr observed in the Cenozoic is applied to the Siberian sections, the calculated minimum duration for the Suram–Wonoka event is 10 Ma. Supplementary material: XRF and ICP-AES analysis, C, O and Sr isotopic data are available at http://www.geolsoc.org.uk/SUP18324.

Isotopic stratigraphy suggests Neoproterozoic ages and Laurentian ancestry for high-grade marbles from the North-Central Norwegian Caledonides

Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician–Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered 87 Sr/ 86 Sr ratios ranging between 0.7062 and 0.7068, and the best preserved δ 13 C values falling between +5.0 and +6.5‰ obtained from two marble formations, are consistent with a seawater composition in the time interval 700–600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician–Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the region requires revision. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica imply that carbonates were initially accumulated in seas on a continental shelf, probably Laurentia, and were tectonically transported onto Baltica during Early Silurian, Scandian collision, at c. 425 Ma. Prospecting for new dolomite marble deposits of the Hekkelstrand type and carbonate-hosted manganese–iron ores should be restricted to 700–600 Ma sequences in the Uppermost Allochthon of the Norwegian Caledonides.

Isotopic evidence for a complex Neoproterozoic to Silurian rock assemblage in the North-Central Norwegian Caledonides

Abstract Depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides have been constrained by the application of carbon and strontium isotope stratigraphy. Several marble units intercalated with various schists of the Evenes Group, previously correlated over long distances farther to the north with a late Ordovician–early Silurian, low-grade, fossiliferous succession (Balsfjord Group), have been studied for carbon, oxygen and strontium isotopes. The least altered 87Sr/86Sr ratios (0.7066 and 0.7077), and the best preserved δ13C values (+5.0 and +8.0‰) obtained from the lowermost and uppermost marble formations of the Evenes Group are consistent with a seawater composition in the Neoproterozoic (650–600 and 620–610 Ma, respectively). The isotopic data indicate that only one formation (87Sr/86Sr=0.7083, δ13C=+5.1‰), forming the middle Evenes Group, is consistent with the previously suggested early Silurian correlation. This is supported by new isotopic data obtained from coral and brachiopod-bearing, Llandovery (443–428 Ma) metalimestones (87Sr/86Sr=0.7083, δ13C=+4.3‰) of the Balsfjord Group. The remaining, fourth formation of the Evenes Group shows isotope data (87Sr/86Sr=0.7088, δ13C=+2.1‰) which are consistent with a Cambrian seawater composition. The results obtained have several implications: (i) the late Ordovician–early Silurian Elvenes Conglomerate/ophiolite assemblage has a tectonic contact with the structurally overlying marble formation dated to 650–600 Ma; (ii) the isotopic and geological data do not support the previously proposed correlation of the entire Evenes Group with a late Ordovician–early Silurian, lithostratigraphic succession farther north in the Balsford area; (ii) the Evenes Group is not a lithostratigraphic unit and should be abandoned, as it is composed of a number of marble formations of different age, tectonically emplaced in a non-chronostratigraphic order. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica suggest that a large part of the carbonate rock succession forming the Evenes Group was initially accumulated in warm seas on a continental shelf, probably Laurentia, and was tectonically transported onto Baltica during the early Silurian, Scandian collision, at ca. 425 Ma. The complex tectonic imbrication of the polymetamorphosed and polydeformed Neoproterozoic, Cambrian and early Silurian carbonate formations also suggests that more than one orogenic episode should be invoked to explain the tectonic juxtaposition of these assemblages. Some of the fault contacts juxtaposing rocks of Neoproterozoic and Cambrian age and the obduction of the ophiolite complex might have been associated with a mid to late Ordovician, Taconian event.

Geochemistry of C, O, and Sr isotopes in the Neoproterozoic carbonates from the southwestern Patom paleobasin, southern Middle Siberia

Scatter of δ13C (from −11.6 to 8.4‰) and sequence of carbon isotope events in the Vendian carbonates from the southwestern Patom paleobasin (Vitim, Bol’shaya Chuya, and Chaya rivers) are similar to those previously established in the reference northeastern sections. It was found that cap dolomites crowning the glaciogenic diamictites of the Dzhemkukan Formation are characterized by moderately low δ13C (from −7.4 to −5.1‰), while the limestones, marls, and calcareous aleurolites of the Zhuya Group (Nikol’skoe and Chencha formations) correlable with the Shuram-Wonoka event (580–650 Ma) have extremely low δ13C values (up to −11.9‰). Intervals with negative δ13C are separated by positive excursion of δ13C (most part of the Barakun, Valyukhta, and Uluntui formations) with extreme up to 8.4‰. One more positive excursion (δ13C up to 6.1‰) was identified in the dolomitic unit sandwiched between the limestones of the Chencha Formation and the sandstones of the Zherba Formation on the Vitim and Bol’shaya Chuya rivers; in the northeastern sections of the Patom Complex, the positive excursion corresponds to the hiatus, but occurs at similar level (∼550 Ma) in many sections of Oman, China, Namibia, Australia, and other regions. Ultrahigh δ13C (from 11.7 to 15.2‰) values were found in ∼150-m-thick succession of “coaly” carbonates in the Lokatyk Block (Bol’shaya Chuya River), which were presumably erroneously ascribed to the Barakun Formation. These carbonates likely marking the Neoproterozoic anoxic peak are not known among the Vendian (Ediakaran) successions, but have previously been described in the sediments with an inferred age of 720–650 Ma in the Bambui Group (eastern Brazil) and Keele Formation of the Neoproterozoic Windermere Group (northwestern Canada). Scatter of δ18O in the studied carbonates (from 6.3 to 28.5‰) was mainly caused by postsedimentary transformations. General correlation between δ18O and δ13C is absent. Both positive and negative correlations can be observed within limited intervals, indicating that the behavior of O and C isotope systems was not controlled by a common mechanism. The lowest 87Sr/86Sr = 0.7084 in the main chemostratigraphic marker (Chencha Formation) in the extreme west (Chaya River) does not differ much from the 87Sr/86Sr value in the Chencha Formation in the northeastern sections, which confirms the hydrological homogeneity of the Patom paleobasin. The lowest 87Sr/86Sr (0.70815) in the carbonates of the Uluntui Formation of the Chaya section, however, is much lower than in the carbonates from the stratotype area (0.70842) of the Uluntui Formation (Kuznetsov et al., 2013). This casts some doubts on the correlation of these stratigraphic units.

Geochemical properties of Neoproterozoic “cap dolomites” in the Patom paleobasin and problem of their genesis

The characteristic feature of many Upper Neoproterozoic glacial sequences is their “cap carbonates” (CC) resting without visible unconformity upon glaciogenic diamictites. Such an unusual association, peculiar structures and textures, and negative δ13C values (approximately −4 ± 2‰) that are atypical of marine carbonates provoked long debates about the nature of these carbonates, which play an important role in the Snowball Earth hypothesis. According to this hypothesis, the Earth was entirely covered by ice during large-scale glaciations, and CC accumulation was related to the global change in geochemical processes. In this work, we discuss data on the chemical and isotopic (C, O, Sr) compositions of CCs, which overlie glacial sediments of the Nichatka and Bol’shoi Patom formations accumulated in different parts of the Neoproterozoic Patom paleobasin (Central Siberia). High concentrations of Fe (up to 6400 ppm), Mn (2320 ppm), and radiogenic Sr (87Sr/86Sr0 up to 0.7172) established in CCs indicates a strong influence of the continental flow. Extraordinary Snowball Earth conditions are not necessary for the accumulation of these rocks, geochemical and sedimentological properties of which may be explained by the discharge of thawing waters into partly or completely isolated near-glacier basin, their intermittent freezing, and/or washout of “frozen” carbonates from the surface of thawing glaciers. The peculiar thin-laminated texture of CC may be related to seasonal processes of climatic cycles. They were accumulated in the course of general (relatively long-term) depletion of the atmosphere and hydrosphere in 13C, which has nothing to do with the CC formation as a specific type of carbonate sediments. Amplitude and duration of the negative δ13C excursion in carbonates associated with the Lower Vendian glacial sediments (665–635 Ma) are appreciably lower than the negative anomaly in rocks of the Zhuya Group that likely correspond to the Shuram-Vonoka Event (∼560−580 Ma ago), which probably marks the crucial point in the Precambrian deglaciation: mass destabilization of methane hydrates and degradation of the Early Vendian psychrosphere in oceans.

Geochemistry of C, O, and Sr isotopes and chemostratigraphy of neoproterozoic rocks in the northern Yenisei Ridge

Isotopic compositions of C, O, and Sr in carbonates, as well as Rb-Sr systems in the silicate material from Upper Precambrian and Lower Cambrian rocks exposed by the Chapa River in the northern Yenisei Ridge, are studied. The Late Precambrian part of the section includes the following formations (from the bottom to top): Lopatinskaya (hereafter, Lopatino), Vandadykskaya (hereafter, Vandadyk) or Kar’ernaya, Chivida, Suvorovskaya (hereafter, Suvorovo), Pod”emskaya (hereafter, Podyom), and Nemchanka. They are characterized by alternation of horizons with anomalously high and low δ13C values (such alternation is typical of the ∼700–550 Ma interval). The lower, relatively thin (20 m), positive excursion (δ13C up to 4.3‰) was established in dolomites from the lower subformation of the Vandadyk (Kar’ernaya) Formation (hereafter, lower Vandadyk subformation). The upper positive excursion (δ13C = 2.2 ± 0.6‰) was recorded in the 3-km-thick Nemchanka Formation enriched in terrigenous rocks. The lower negative excursion stands out as uniform, moderately low δ13C values (−2 ± 1‰) and significant thickness. It comprises the upper part of the Vandadyk Formation, as well as Chivida and Podyom formations. The upper negative excursion is related to a thin (∼20 m) marker carbonate horizon of the upper Nemchanka subformation, in which δ13C values fall down to −8.3‰. The lower part of the Lebyazhinskaya (hereafter, Lebyazhino) Formation, which overlies the Nemchanka Formation, shows a step-by-step increase in δ13C from −2.2 to 2.5‰ typical of the Vendianto-Cambrian (Nemakit-Daldyn Horizon/Stage) transitional sequences. The absence of relationships between the carbon and oxygen isotope compositions and other parameters of postsedimentary alterations suggests that the excursions characterized above could form at the sedimentation stage and coincide in general with δ13C fluctuations in seawater. The value of 87Sr/86Sr = 0.7076−0.7078 in limestones of the Podyom Formation points to their early Ediacaran age. Values of 87Sr/86Sr = 0.70841 and 0.70845 in dolomites of the lower Lebyazhino subformation correspond to the Early Cambrian. The Rb-Sr systems of the clay material from the Vandadyk and Chivida formations are approximated by a straight line, parameters of which correspond to the age of 695 ± 20 Ma (87Sr/86Sr0 = 0.7200 ± 0.0013) and probably characterize the epigenetic stage of older sedimentary rocks, which were subjected to very rapid exhumation and “polar” sulfuric acid weathering in the course of glacioeustatic regression.

Waters from mud volcanoes of Azerbaijan: Isotopic-geochemical properties and generation environments

New data on chemical and isotopic properties of waters from 35 mud volcanoes of Azerbaijan show that they are represented by two contrasting types characterized by different mineralization: Cl-Na (M = ∼30–80 g/L) and HCO3-Cl-Na (M = ∼8–15 g/L). Waters of the last type are usually enriched with alkaline metals, B, and Br. Waters of the Cl-Na type are similar in their Cl/Br coefficient with seawater. According to Mg-Li and Na-Li geothermometers, waters were formed at temperatures varying from ∼20 to ∼140°C. Taking into consideration these estimates, “roots” of the volcanoes under consideration should be located at depths of 6.0–7.5 km. The fluid generation temperatures exhibit a tendency for their growth toward the Greater Caucasus orogen. The δD, δ18O, and δ13C (TDIC) values in dissolved inorganic carbon vary from −32 to −12 and −0.6 to +10.4‰ (V-SMOW) or −12.9 to +37.3‰ (V-PDB), respectively. The highest δD, δ18O, and δ13C values are typical of HCO3-Na waters. It is shown that the growth of formation (Mg-Li) temperatures is accompanied by a significant increase of the δ18O(H2O) and δ13C(HCO3−) values along with HCO3− concentrations. The degree of water enrichment with Br, B, and alkaline metals also increases. This allows the formation of soda waters from mud volcanoes with elevated 18O and 13C contents to be attribute to relatively high-temperature transformations of mineral and organic matter during the catagenic alteration sedimentary rocks.

Sr chemostratigrphy, δ13C, and δ18O of rocks in the Crimean carbonate platform (Late Jurassic, northern Peri-Tethys)

The paper presents the results of study of the Sr, C, and O isotope compositions in Upper Jurassic carbonate rocks of the Baidar Valley and Demerdzhi Plateau in the Crimean Mountains represented by different facies of the carbonate platform at the northern active margin of the Tethys. The 87Sr/86Sr value in them varies from 0.70699 to 0.70728. Based on the Sr chemostratigraphic correlation, the age of massive and layered limestones in the western part of the Ai-Petri and Baidar yailas (pastures) is estimated as late Kimmeridgian–early Tithonian, whereas the age of flyschoids of the Baidar Valley are estimated as late Tithonian–early Berriasian. The nearly synchronous formation of carbonate breccias of the Baidar Valley and Demerdzhi Plateau in late Tithonian–early Berriasian is substantiated. A summary section of Upper Jurassic rocks is compiled based on the Sr chemostratigraphic data. It has been established that δ18O values in the studied carbonate sediments vary from–2.9 to 1.3‰ (V-PDB). At the same time, shallow-water sediments in the internal part and the edge of the Crimean carbonate platform are depleted in 18O (–2.9 to +0.1‰) relative to sediments on the slope and foothill (–0.5 to +1.3‰). It is demonstrated that δ13C values do not depend on the facies properties and decrease in younger carbonate sediments from 3–3.5‰ to 1–1.5‰ in line with the Late Jurassic general trend. The δ13C values obtained for the Crimean carbonate platform turned out to be 0.5–1‰ higher than the values typical of the deep-water marine setting at the western margin of the Tethys. These discrepancies are likely related to peculiarities of water circulation and high bioproductivity in marine waters of the northern Peri-Tethys.

Sedimentary environments and geochemistry of Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus

Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus were examined in the most representative Chirkei section (Sulak River basin). Sharp lithogeochemical distinctions between them were revealed. The results of the study of nannoplankton demonstrated that the Eocene/Oligocene interface occurs slightly below the boundary between the Belaya Glina and Khadum formations. The studied section revealed a series of nannoplankton bioevents facilitating its stratigraphic subdivision. It has been established that organic matter (OM) in rocks of the Khadum Formation is characterized by a relatively high degree of maturity. Probably, the material of mainly marine genesis contains a terrigenous OM admixture. Positive oxygen isotope anomaly in the upper part of the Belaya Glina Formation reflects global climate changes (cooling) near the Eocene/Oligocene interface. Limitation of the anomaly by the upper boundary of the Belaya Glina Formation is likely related to changes in water salinity variations in the Early Oligocene basin and intense early diagenetic processes in rocks therein. Lithological, geochemical, and paleoecological data suggest that the Khadum paleobasin was depleted in oxygen. Such environment was unstable with periodic intensification or attenuation. Paleoecology in the Belaya Glina basin was typical of normally aerated basins.

Strontium and oxygen isotopic systems in waters of mud volcanoes of the Taman Peninsula

Ten of eleven analyzed water samples from mud volcanoes of the Taman Peninsula are characterized by 87Sr/86Sr ratio within 0.70734–0.70957, which overlaps the values typical of the Mesozoic and Cenozoic sedimentary carbonates, but sharply differs from the value in the clayey sediments of the Maikop Group (0.7157 ± 0.0022). These data indicate that the strontium isotopic composition is mainly defined by carbonate reservoirs, with relatively little effect of elision solutions, input of which is noticeable only in the water of Gladkovsky Volcano (87Sr/86Sr = 0.71076). The high δ18O in mud volcanic waters (up to 14.2‰) can also be attributed to ionic exchange with sedimentary carbonates at temperatures around 150°C.